Synthesis of o-Methyl Trifluoromethyl Sulfide Substituted

Apr 5, 2017 - An efficient process for the preparation of valuable o-methyl trifluoromethyl sulfide substituted benzophenones has been developed. The ...
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Synthesis of o‑Methyl Trifluoromethyl Sulfide Substituted Benzophenones via 1,2-Difunctionalization of Aryne by Insertion into the C−C Bond Milind M. Ahire, Ruhima Khan, and Santosh B. Mhaske* Division of Organic Chemistry, CSIR-National Chemical Laboratory, Pune 411 008, India S Supporting Information *

ABSTRACT: An efficient process for the preparation of valuable o-methyl trifluoromethyl sulfide substituted benzophenones has been developed. The transition-metal-free method features insertion of aryne into a C−C σ-bond under mild reaction conditions for the first time to achieve ortho-difunctionalized arenes containing a pharmaceutically important trifluoromethylthio functional group. A wide substrate scope has been demonstrated for the developed protocol.

F

luorinated compounds constitute a vital structural class, which is commonly found in bioactive molecules, agrochemicals, polymers, and materials.1 Until now, more than 20% of modern pharmaceutical ingredients are known to contain a fluorine atom, which highlights its importance in new drug design and development.1a Due to fluorine’s intrinsic property of modifying the pharmacological and physicochemical properties of drug molecules, its installation has become an important objective. Hence, in the past few years, methods involving efficient incorporation of fluorine and fluorine-containing moieties into organic compounds have gained tremendous attention from the scientific community. 2 Among the fluorinated moieties, trifluoromethylthio (−SCF3) has experienced long-standing attention for its unique biological properties such as extremely high Hansch lipophilicity parameter, protein binding affinity, metabolic stability, and strong electron-withdrawing effect, which allows high permeability of drug candidates through the lipid membrane to exert their effects.3 Hence, novel strategies for the preparation of −SCF3-containing compounds are always sought after for designing new drugs.2a,b,4 As shown in Figure 1, some biologically active molecules contain −SCF3 as an important pharmacophore.5 The literature review revealed that a vast number of synthetic methods for the introduction of a trifluoromethylthio group on the aromatic ring have been well developed.2a,b,4 Transitionmetal-catalyzed C−H activation, cross-coupling reactions of aryl halides or aryl boronic acids with −SCF3-containing reagents, and electrophilic and nucleophilic substitution reactions are some of the important methods.2a,b,4 However, very little consideration has been given on the aliphatic −SCF3 bond-formation reactions.4b The known synthetic methods largely deal with the preparation of trifluoromethylthiolation by means of prefunctionalized starting materials.4,6 These methods consist of nucleophilic displacement of good leaving groups by −SCF3. However, a method implying the addition of α-SCF3 ketones on aryne to form novel compounds with additional © 2017 American Chemical Society

Figure 1. Bioactive compounds containing −SCF3.5

functionality on organic molecules has not been reported to date. Herein, we developed a mild synthetic strategy for ketone derivatives (particularly benzophenones) with additional orthobenzylic trifluoromethylthio functionality using stable and easy to handle α-SCF3 ketones and arynes generated by Kobayashi’s method.7 Arynes are very reactive intermediates and have fascinated synthetic chemists for quite a long time.8 Because of its high electrophilicity, it has been involved in the development of many new synthetic methodologies. Generally, aryne has been trapped by various nucleophiles in assembling new carbon− carbon and carbon−heteroatom bonds for efficient construction of a diverse array of useful synthetic building blocks.8 In addition, aryne has been successfully applied for designing valuable ortho-disubstituted arenes by dipolar cylcoaddition and multicomponent reactions (MCR).8g,9 Notably, for 1,2disubstituted arenes, aryne insertion into an element−element σ-bond is a considerable transformation, and varieties of Received: March 15, 2017 Published: April 5, 2017 2134

DOI: 10.1021/acs.orglett.7b00768 Org. Lett. 2017, 19, 2134−2137

Letter

Organic Letters Table 1. Optimization of Reaction Conditionsa,b

substrates have been studied with or without transition-metal catalysts.10 Our group has been involved in developing novel synthetic methodologies involving arynes.11 Inspired by the importance and the recent progress in the area of aryne insertion reactions, we have developed a new synthetic methodology for the expedient synthesis of o-CH2SCF3substituted benzophenones. In contrast to most of the previous reports, this newly developed aryne insertion reaction takes place on heteroatom-substituted methylene ketone under much milder conditions (Figure 2).12

entry

solvent

F− source

temp (°C)

additive

time (h)

5ac (%)

1 2 3 4 5 6d 7 8

THF toluene CH3CN THF THF THF toluene CH3CN

KF, 18-c-6 KF, 18-c-6 CsF KF, 18-c-6 KF, 18-c-6 KF, 18-c-6 KF, 18-c-6 CsF

rt rt rt rt 0 0 0 0

Cs2CO3 Cs2CO3 Cs2CO3 − − − − −

0.5 12 2 1 1 2 12 3

63 60 45 60 85 67 70 51

a Selected entries. bReaction conditions: 1a (0.34 mmol, 1.5 equiv), 4a (0.23 mmol, 1 equiv), Cs2CO3 (1.2 equiv), CsF (3.0 equiv)/KF (3.0 equiv), and 18-crown-6 (3.0 equiv) in solvent (1 mL). cIsolated yield. d 1a (1 equiv), 4a (1 equiv).

source in THF at room temperature. Our initial hypothesis behind using a base for generating active carbanion species was ruled out after the reaction was performed in the absence of Cs2CO3. The comparable yield (Table 1, entry 4) obtained for the product 5a without any base determines the role of excess KF as a base for this transformation. The shorter time required for the completion of the reaction illustrates the faster reaction rate; hence, the same reaction was executed at 0 °C, which provided 5a with increased yield up to 85% (Table 1, entry 5). Furthermore, the variation in the molar ratio of the substrates 1a and 4a was also examined. Reduction in the yield was noticed with reduced aryne precursor equivalents (Table 1, entry 6). Similarly, performing the reaction in toluene and acetonitrile at 0 °C (Table 1, entries 7 and 8) furnished 5a in 70% and 51% yield, respectively, with a mixture of side products; thus, THF was found to be the best solvent for this transformation. The reproducibility of the optimized reaction protocol (Table 1, entry 5) at a higher scale was confirmed by performing the reaction on the substrate 4a on a 1 mmol scale, which furnished the product 5a in 70% yield. With the optimized reaction conditions in hand, we investigated the substrate scope of this newly developed protocol by varying the silyl triflates (1a−h). A variety of electron-donating and electron-withdrawing groups on silyl triflates were tested, and the corresponding products were obtained in good to moderate yields (Scheme 2, entry 5a−h). As mentioned above in the optimization study, the unsubstituted silyl triflate 1a provided the expected product in 85% yield. However, surprisingly simple alkyl-substituted aryne precursors 1b and 1c did not react at 0 °C, and less product formation was observed even at room temperature. Improved yield was observed only when additional base Cs2CO3 was added at 0 °C followed by stirring the reaction mixture at room temperature. The reason behind this observation is obscure. Remarkably, for the unsymmetrical aryne precursor 1d, we were pleased to observe the expected product formation with only one regioisomer 5d. The observed regioselectivity might be due to the low reaction temperature and electronic effects of the methoxy group. It is noteworthy that electron-rich substrates 1e and 1g furnished the expected products 5e and 5g, respectively, in good chemical yields. However, aryne

Figure 2. Previous aryne insertion reactions into C−C and C−X bonds and this work.12

Our investigation of insertion reactions began with the treatment of benzyne generated from 2-(trimethylsilyl)phenyltriflate (1a) with α-SPh ketone 213 in the presence of KF, 18-crown-6 ether, and Cs2CO3 in THF under argon atmosphere at room temperature (Scheme 1). Gratifyingly, we Scheme 1. First Reaction Attempted

observed the formation of the expected insertion product 3 in 60% yield. Encouraged by this result and taking into consideration the importance of fluorine-containing organic compounds, we envisaged that this process will be more useful to insert aryne into the C−C σ-bond of the α-SCF3 ketones to obtain more value-added products. Hence, we prepared α-SCF3 ketone 4a by the reported procedure14 and applied the abovementioned aryne insertion reaction conditions. Interestingly, 63% yield of the expected product 5a was observed within 30 min (Table 1, entry 1). Initially, to attain optimized reaction conditions, we screened effective fluorinating sources along with corresponding solvents. The outcome of the entries 1−3 (Table 1) led us to conclude that KF and 18-crown-6 ether stood out to be the best fluoride 2135

DOI: 10.1021/acs.orglett.7b00768 Org. Lett. 2017, 19, 2134−2137

Letter

Organic Letters Scheme 2. Reaction with Various Silyl Triflatesa,b

without difficulty. The substrates 4e and 4f having halo groups (F and Br, respectively) provided the products 6e and 6f in good to excellent yields, respectively. These products can also be further derivatized by coupling reactions. Furthermore, the substrate 4g having an electron-withdrawing group (p-SO2Ph) furnished the relevant product 6g smoothly with decent yield. We were pleased to find that the developed conditions worked very well on naphthyl substrate 4h to provide the corresponding product 6h in 70% yield. The developed process was also successfully employed on heterocyclic ketone 4i containing a thiophene moiety to obtain 6i with good yield. In conclusion, we have developed a convenient and a novel method for the preparation of ortho-difunctionalized arenes having a trifluoromethylthio functional group by aryne insertion to α-SCF3 ketones. The reaction tolerates a variety of substituents on arynes as well as ketones. This methodology allows metal-free access to a range of value-added aromatic ketones bearing an o-CH2SCF3 group. Furthermore, we are in the process of screening the final compounds for potential biological activity and currently focusing on the development of novel methodologies for aryne insertion into elemental− elemental bonds and their application in the synthesis of bioactive molecules and natural products.

a

Reaction conditions: 1a−h (0.34 mmol,1.5 equiv), 4a (0.23 mmol, 1 equiv), KF (3.0 equiv), and 18-crown-6 ether (3.0 equiv) in THF (1 mL). bIsolated yield. crt, Cs2CO3 (1 equiv).

precursor 1f was unreactive at 0 °C and formed complex mixture of products at room temperature. The desired product 5f was observed, though in low yield, when the reaction was performed at 15 °C for 1 h. Further stirring the reaction mixture for a longer time again showed formation of complex reaction mixture. The difluoro-substituted aryne precursor 1h provided a good yield of the desired product 5h under the optimized conditions. After understanding the reactivity pattern on different aryne precursors, we turned our attention toward study of the substrate scope by varying the α-SCF3 ketones (4b−i) and keeping constant the aryne precursor 1a. Various electrondonating and electron-withdrawing substituents on the phenyl ring of α-SCF3 ketones were examined. The corresponding inserted products were obtained with good to moderate yields (Scheme 3, 6b−i). The substrate 4b reacted smoothly to provide the aryne-inserted product 6b in very good yield. However, the ketone 4c remained unreactive at 0 °C as well as at room temperature. Hence, we used a base Cs2CO3, which can abstract the methylene protons easily, and to our delight, the product 6c was obtained in good yield. The p-phenylsubstituted ketone 4d furnished the corresponding product 6d Scheme 3. Reaction with Various α-SCF3 Ketones

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b00768. Experimental procedures and spectroscopic data of all new compounds (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Santosh B. Mhaske: 0000-0002-5859-0838 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.M.A. thanks CSIR-New Delhi for the research fellowship. S.B.M. gratefully acknowledges generous financial support from DST and CSIR-ORIGIN, New Delhi.

a,b

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Reaction conditions: 1a (1.5 equiv), 4b−i (50 mg, 1 equiv), KF (3.0 equiv), and 18-crown-6 ether (3.0 equiv) in THF (1 mL). bIsolated yield. cCs2CO3 (1.0 equiv). 2136

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DOI: 10.1021/acs.orglett.7b00768 Org. Lett. 2017, 19, 2134−2137